An alkaline chloride electrolytic process employing an ion exchange membrane method, which comprises electrolyzing an aqueous alkaline chloride solution using an ion exchange membrane as a diaphragm to produce an alkali hydroxide and chlorine, has been known.
As an ion exchange membrane, usually, a membrane made of a polymer having sulfonic acid groups or a laminate having a layer made of a polymer having sulfonic acid groups is used. As the polymer, the following polymer is used for the general purpose.
(1) A polymer obtained by subjecting a copolymer of the compound represented by the following formula (1) with tetrafluoroethylene to hydrolysis to convert —SO2F groups to sulfonic acid groups (—SO3M groups) (Patent Document 1).CF2═CF—(OCF2CFY)m—Op—(CF2)n—SO2F  (1)wherein M is a hydrogen atom or an alkali metal, Y is a fluorine atom or a trifluoromethyl group, m is an integer of from 0 to 3, n is an integer of from 1 to 12, and p is 0 or 1, provided that m+p>0.
An ion exchange membrane has been desired to produce an alkali hydroxide and chlorine at a small electric power for electrolysis (low electrolysis voltage). Accordingly, as a polymer to be used for an ion exchange membrane, a polymer having a low electrical resistance i.e. a low equivalent weight has been desired. In order to reduce the equivalent weight of the polymer (1), the amount of sulfonic acid groups, namely, the proportion of the compound represented by the formula (1), may be increased.
However, if the proportion of the compound represented by the formula (1) is increased, it tends to be difficult to sufficiently increase the molecular weight of the polymer, and further, the polymer will be excessively swollen with water, whereby the mechanical strength of the ion exchange membrane will be insufficient.
Patent Document 1: JP-A-6-33281